1. Field of the Invention
The present invention relates to a demodulation circuit which is used in a receiver, such as a mobile radio transceiver, and more particularly, to an improvement in a demodulation circuit which has an automatic frequency control (AFC) function of automatically controlling the frequency of a received signal having an intermediate frequency dropped from an original frequency of the received signal within a predetermined frequency range in order to maintain the signal to be demodulated at a high quality.
2. Description of the Related Art
FIG. 6 is a block diagram of an example of a prior art demodulation circuit of the type referred to. The demodulation circuit of FIG. 6 includes a mixer 1 for mixing input signals having different frequencies and outputting a mixture signal thereof, first and second band pass filters 2 and 4 for passing therethrough only signals in a predetermined frequency range, a quadrature demodulator 3 for demodulating an intermediate frequency signal passed through the first pass filter 2, first and second reference oscillators 5 and 6 (second reference oscillator 6 being of a variable frequency type) each comprising, e.g., a crystal oscillator for oscillating a reference signal having a reference frequency for demodulation of a received signal, a synthesizer 7 for generating and outputting a signal having a predetermined frequency on the basis of the reference signal oscillated at the first reference oscillator 5, a matched filter unit 8 for correcting the oscillation frequency of the second reference oscillator 6 so that an input signal has a maximum power value, an antenna 10 for receiving radio waves, and a modem 11 for converting a signal demodulated through the quadrature demodulator 3 as a base band signal into, e.g., a digital signal and then for performing a predetermined modulating/demodulating operation over the base band signal.
With such a demodulation circuit as mentioned above, when the antenna 10 catches radio waves, the caught waves are applied to the mixer 1 as a high frequency signal 100 and then mixed in the mixer 1 with a signal 200 having a predetermined frequency received from the synthesizer 7.
As a result of the mixing operation, an intermediate frequency signal 300 having an intermediate frequency of, for example, about 80 MHz is output from the mixer 1. The intermediate frequency signal 300 is subjected at the first band pass filter 2 to a filtering operation and then sent to the quadrature demodulator 3.
The quadrature demodulator 3 functions to mix the received intermediate frequency signal 300 and a reference signal oscillated at the second reference oscillator 6 of the variable frequency type to generate a base band signal 400. The demodulated base band signal 400 is subjected at the second band pass filter 4 to a predetermined filtering operation and then applied to the modem 11 and also to the matched filter unit 8.
The matched filter unit 8, as explained above, is used to check the power value of the input signal 400 and to correct the oscillation frequency of the second reference oscillator 6 in such a manner that the input signal 400 has a maximum power value. In more detail, the matched filter unit 8:
(1) Checks the power value of the input signal 400.
(2) Outputs a control signal 50 for shifting the frequency of the oscillation signal of the second reference oscillator 6 by such an amount that causes shift of the frequency of the signal 400 by a small frequency amount of .DELTA.f.
(3) Re-checks the power value of the signal 400 having a frequency shifted by the amount of .DELTA.f and compares the current and previous signals 400 with respect to their power values.
(4) When the comparison proves an increase in the power value, the unit 8 repetitively performs the above processing (2) to shift the frequency of the signal 400 in the same direction as above and performs the above comparing operation (3).
(5) When the comparison proves a decrease in the power value, the unit 8 re-performs the above processing (2) to shift the frequency of the signal 400 in the opposite direction from above and performs the above comparing operation (3).
That is, the matched filter unit 8 repetitively carries out such operations as mentioned above.
The signal 400 demodulated as the base band signal and then applied to the modem 11 in this way always has a maximum power level and is maintained at a high level in quality.
Such an automatic frequency control (AFC) function is useful for enhancing and maintaining the quality of the demodulated signal. However, the aforementioned prior art circuit has a defect that, when such a change in the oscillation frequency output from the first reference oscillator 5 causes fluctuations of the frequency of the intermediate signal 300, the normal AFC function of this circuit cannot be ensured.
In more detail, since the aforementioned frequency correction of the matched filter unit 8 provides a definite shift width, if the frequency of the intermediate frequency signal 300 exceeds the correctable range of the matched filter unit 8 for some reason, then the demodulation circuit cannot accurately detect the maximum power value of the signal 400 and thus the AFC function thereof cannot operate normally.
The above defect can be avoided by enlarging the shift width for the frequency correction of the matched filter unit 8, but this requires increase of the number of gates to be used in the matched filter unit 8, which results in that the circuit becomes complicated and expensive.